1. Field of the Invention
The present invention relates generally to lighting control systems and the regulation of lighting loads to maintain a constant light level in order to reduce energy use when natural light is available which process is generally known as daylighting or daylight harvesting.
2. Description of the Related Art
Daylighting is simple in principal but has traditionally been problematic in execution. The basic concept is to maintain a constant light level or setpoint in a space in the same way that a thermostat maintains a constant temperature setpoint.
Light sources in a given space are either controlled or uncontrolled. Uncontrolled light is generally contributed by natural sources like the sun while controlled sources are electric lights or mechanisms that can be operated to modulate the amount of light coming into a space. As the light level from uncontrolled sources changes the daylighting system increases or decreases the output of the controlled sources to maintain a constant light level. This is done by using either open or closed loop control. Open loop operates without feedback while closed loop has feedback. Open loop has the advantage of not being affected by local conditions such as window shades and changing surface reflectance of room surfaces while closed loop is affected by local conditions. In areas like public spaces where local conditions are effectively static open loop can be effective. However, in offices, classrooms, and other spaces where room and use conditions are not static, closed loop control is needed to be able to respond to changing conditions and user needs.
Thermostats are a good example of a closed loop system. The thermostat measures temperature in a space and then compares that temperature to a setpoint value. If the thermostat is controlling a heat source a measured temperature greater than the setpoint means the space is too hot and the heat source is turned off or reduced. If the temperature is below the setpoint then the heat source is activated or increased to raise the temperature. Daylighting works much the same way by comparing a measured light level against a light level setpoint. If the light level is above the setpoint the controllable light source is decreased and if it is below the setpoint the light source is increased. Excessive cycling in both systems is managed by establishing a dead band around the setpoint that creates a window within which no adjustment occurs.
However, in other ways heat and light are very different. Air temperature is slow to respond to heat or cooling sources and given a space with reasonable air circulation, air temperature will be about the same throughout the space. Light, however, is directional and immediate so light level can change quickly and vary widely depending on where it is measured. Consequently, light sensors must be placed in the area they are controlling and then calibrated for that specific location.
All things being equal, the best placement for a daylighting light sensor is at the work surface of the zone being controlled. However, for several reasons this is generally not practical. The next best location is typically on the ceiling where the sensor is protected and has a good field of view. In theory this works well except that a ceiling mounted light sensor relies on reflected light and is generally inaccessible to users. Reflected light is a problem because it depends on the reflectance of room surfaces which can change quickly with user activity. Inaccessibility is a problem because it limits the user's ability to tune the system to meet changing use and environmental conditions. The user point of control could be moved to a more accessible location but this is typically considered too expensive and setting a light level is not as intuitive as setting a temperature setpoint.
Prior art daylighting systems have managed this problem by providing a work around. Instead of allowing users to adjust the light level setpoint directly the solution has been to provide a fixed setpoint together with an independent means for users to control light level. With dimmable light sources users are free to adjust their lights to any level within the dimming range. However, user adjustable light levels are inherently incompatible with basic fixed setpoint daylighting. Prior-art, closed loop daylighting systems have solved this problem by creating an asymmetrical dead band around the fixed setpoint. Daylighting engages whenever the measured light level is greater than the setpoint but daylighting below the setpoint is eliminated. This gives users the freedom to set a light level to any value below the setpoint but at the cost of eliminating daylight harvesting until the light level increases above the setpoint.
Ignoring the loss in energy savings, prior art daylighting has several additional problems. The first is light sensor calibration. Occupied work spaces are rarely static. Furniture is moved, clothing changes, desk surfaces become cluttered, and light sources deteriorate and fail. All these factors affect the amount of light reaching the light sensor. A good example is a room with dark carpet and moveable white desks. Moving the desks can quickly and significantly change the room reflectance which in turn can have an immediate and significant effect on the amount of light reaching the light sensor. Prior art systems typically manage this problem by providing a means for a maintenance person to reset and recalibrate the system but this approach is both inconvenient for users and expensive for maintenance departments.
A second major problem is light sensor placement. Prior art daylighting systems work by maintaining a static setpoint. This works well as long as the light sources are also static. However, daylight comes from the sun and the sun position changes with the seasons. A light sensor calibrated for the winter can be overwhelmed in summer. The prior art solution has been to locate the sensor so that the daylight contribution is relatively constant over the year. To aid in this calculation the California Energy Commission funded development of a light sensor placement program called SPOT which stands for Sensor Placement Optimization Tool. The tool is free and downloadable from the Internet but modeling is time intensive and challenging. Instead, sensors are typically located by general placement guidelines which are often not well understood or easy to enforce.
Sensor placement is also restricted by bleed light from non-daylighting zones. Sensors in smaller rooms with only a single control zone are not a problem but in larger spaces like classrooms daylighting and non-daylighting zones can be side by side. Light bleeding into the daylighting zone from a non-daylighting zone will affect daylighting operation. Light sensors can be adjusted to accommodate bleed light but only if the bleed light source is constant. However, in systems with user adjustable light levels bleed light is not constant. Prior art systems work around this problem by requiring the light sensor to be placed where bleed light into the controlled zone is insignificant.
These two placement restrictions are difficult enough but there are still further restrictions. These include non-physical items like the upward light from pendant fixtures as well as the numerous physical items that populate a ceiling including light fixtures, fire sprinklers, air registers, and audio visual projectors. Taken together, finding space to properly locate a prior art light sensor can be challenging.
Thusly, what is needed is a new approach that maximizes daylighting energy savings, quickly responds to changing room conditions, has fewer sensor placement restrictions, and operates with minimal maintenance support.